1. Field of the Invention
The present invention is related to a broadband circuit shorted resonant patch antenna, and especially to a patch antenna for which a resonant circuit is used to compensate its available frequency under the situation that the set length of the patch antenna is shortened.
2. Description of the Prior Art
A patch antenna is made mainly from an extremely thin foil (such as a copper foil) and is in the form of a planar antenna; it is used such as on a movable communication instrument in lieu of a prolonged antenna. Generally, a circuit shorted resonant patch antenna radiates taking advantage of the electric field distribution on the open circuit end thereof.
As shown in FIG. 1 which shows a conventional broadband circuit shorted resonant patch antenna 10, the antenna 10 is provided with an open circuit end 11 and a short circuit end 12, and with a set transverse length xe2x80x9cLxe2x80x9d. The open circuit end 11 is provided with open slots 13, 14 of which an electric field radiates. Basically, such a circuit shorted resonant patch antenna 10 has the best condition for radiation when the energy of electric waves resonates in the patch antenna 10.
FIGS. 2 and 3 show an electric field and a diagram of electric current distribution respectively. We can see from the drawings that, electric current strength is the largest at the open circuit end 11, and is the smallest at the short circuit end 12. In fact, the above stated electric current distribution is only a part of the resonance waves shown in FIG. 4.
Utilizing the above stated concept of resonance waves, the length xe2x80x9cLxe2x80x9d of the patch antenna 10 can be conveniently set as below:   L  =            0.49      2        xc3x97          λ                                    ϵ            r                          ⁢                  xe2x80x83                    
In the formula, ∈r is a dielectric constant; xcex is wavelength. An antenna of half of the wavelength long is divided into two; thereby, it shall be divided by 2. By the nature that the electric field at the center of the resonant electric current is zero, the center of the patch antenna 10 can be grounded to form a single slot radiation, and H and E planar electric fields has the formulae as below:
Excex8=Eo cos xcfx86f (xcex8,xcfx86)
Excfx86=Eo cos xcex8sin xcfx86f (xcex8,xcfx86),
wherein,       f    ⁡          (              θ        ,        φ            )        =                    sin        ⁡                  [                                                    β                ⁢                                  xe2x80x83                                ⁢                W                            2                        ⁢            sin            ⁢                          xe2x80x83                        ⁢            θsinφ                    ]                                                  β            ⁢                          xe2x80x83                        ⁢            W                    2                ⁢        sin        ⁢                  xe2x80x83                ⁢        θsinφ              ·          cos      ⁡              (                                            β              ⁢                              xe2x80x83                            ⁢              L                        2                    ⁢          sin          ⁢                      xe2x80x83                    ⁢          θcosφ                )            
Wherein, xcex2 is a free-space phase constant.
According to the above stated conventional structure of a broadband circuit shorted resonant patch antenna, the distribution diagram of current xe2x80x9caxe2x80x9d when power is turned on is shown in FIG. 5. This diagram shows that resonance of a resonance electric current of such a patch antenna is determined by the length xe2x80x9cLxe2x80x9d of the antenna. A conventional broadband circuit shorted resonant patch antenna has its length xe2x80x9cLxe2x80x9d of the antenna set according to the above listed formula, it is thereby hard to be shortened, hence the whole patch antenna can hardly be miniaturized.
The object of the present invention is to provide a broadband circuit shorted resonant patch antenna of which the length can be shortened under an identical resonance frequency.
To get the above stated object, circuit shorted resonant patch antenna of the present invention is controlled to extend the path of electric current and to determine the resonance frequency thereof. Lengthening of the path of electric current can lower the resonance frequency; thereby the length of the patch antenna can be shortened. In cooperation with the design of shortening the length of the patch antenna, a compensating device can perform broadband compensation to maintain the bandwidth in use.
In a preferred embodiment of the present invention, the above stated short circuit end can be partially shorted to extend the path of electric current. And the broadband compensating device mentioned above can form a resonance circuit with a capacitor and an inductor parallelly connected therewith.
The broadband compensating device mentioned above can further use a transmission line at the open circuit end of its neighboring patch antenna.
The present invention will be apparent in its novelty and other characteristics after reading the detailed description of the preferred embodiment thereof in reference to the accompanying drawings.